PhotosynthesisChapter 10

Photosynthesis• Process that converts solar energy into chemical

energy• Directly or indirectly, photosynthesis food for

almost the entire living world

PhotosynthesisPhotosynthesis is a complex series of reactions that can be summarized as the following equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

Autotrophs= produce own food

Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

Almost all plants are photoautotrophs Uses the energy of sunlight

to make organic molecules

Photosynthesis• Initial Source of Energy for

Most Systems• Primary Producers= source of

organic molecules for most other organisms– Terrestrial Systems= Plants– Aquatic Systems=

Phytoplankton• Consumers= heterotrophs=

consume organic material from other organisms– Cellular Respiration

Primary Producers

Consumers

Decomposers

Photosynthesis• Plants use ~50% of sugars

produced for cellular respiration to fuel own cellular processes

• Rest of sugar is used as building blocks for other organic molecules

• Ultimately, consumed by heterotrophs and used for cellular respiration

(a) Plants(b) Multicellular

alga

(c) Unicellularprotists

(d) Cyanobacteria

(e) Purple sulfurbacteria

10 m

1 m

40 m

Figure 10.2

Primary Producers: plants, algae, protists, and some prokaryotes

These organisms feed not only themselves but also most of the living world.

Heterotrophs are the consumers of the biosphere Heterotrophs= obtain organic material from other

organismsAlmost all heterotrophs, including humans, depend

on photoautotrophs for food and O2

Consumers

Humans rely on primary producers for other types of fuel• The Earth’s supply of fossil fuels was formed from the remains of

organisms that died hundreds of millions of years ago– In a sense, fossil fuels represent stores of solar energy from the

distant past• Biofuels

Chloroplasts• Energy-producing organelle in

plants• Converts solar energy into

chemical energy

– Contains green pigment known as chlorophyll

• Light absorbing pigment• Gives leaves the green color

– All green parts of plant have chloroplasts

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

– Cells can have 30-40 chloroplasts

Mesophyll

Leaf cross section

Chloroplasts Vein

Stomata

Chloroplast Mesophyllcell

CO2 O2

20 mFigure 10.4a

Outermembrane

IntermembranespaceInnermembrane

1 m

Thylakoidspace

ThylakoidGranumStroma

ChloroplastFigure 10.4b

Photosynthesis• Photosynthesis is a complex series of reactions

that can be summarized as the following equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Figure 10.5

Reactants:

Products:

6 CO2

6 H2O 6 O2

12 H2O

C6H12O6

Photosynthesis

Photosynthesis• Endergonic reaction

– Sunlight• Energy stored in bonds of glucose• Redox Reaction

– Photosynthesis reverses the direction of electron flow compared to respiration

– Oxidation= loss of electrons• Water is oxidized

– Reduction= gain of electrons• Carbon dioxide is reduced

Figure 10.UN01

Energy 6 CO2 6 H2O C6 H12 O6 6 O2

becomes reduced

becomes oxidized

Photosynthesis

Photosynthesis occurs in 2 stages• Light Reactions (“photo)

– Occurs in thylakoids– Split H2O

– Release O2

– Reduce NADP+ to NADPH– Generate ATP from ADP by photophosphorylation

Light

LightReactions

Chloroplast

NADP

ADP

+ P i

H2O

Figure 10.6-1

Light

LightReactions

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O

O2

Figure 10.6-2

Photosynthesis occurs in 2 stages

• Dark Reactions: Calvin Cycle (“synthesis”)– Occurs in the stroma

• Uses ATP and NADPH from light reaction to reduce CO2 and produce sugars

• Begins with carbon fixation, incorporating CO2 into organic molecules

Light

LightReactions

CalvinCycle

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

Figure 10.6-3

Light

LightReactions

CalvinCycle

Chloroplast

[CH2O](sugar)

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

Figure 10.6-4